K. Chang has developed an antibody (Kl) that reacts with many ovarian cancers and mesotheliomas. The antigen (CAK1) is a 40 Kd glycosylated protein attached to the cell surface by a PI anchor. To obtain the gene encoding this antigen, he has screened a lambda expression library using antibodies, isolated clones and sequenced these clones. The protein encoded by these cDNAs is a cytosolic protein sharing an epitope with CAK1 but is probably not the membrane protein. Nevertheless, this antigen is expressed in the same cells as CAK1 and may be a member of the same family. Using an alternative approach, he has recently obtained new clones which have the characteristics of the membrane form of the CAK1 antigen. Dr. Chang has also compared the reactivity of monoclonal antibody B3 with that of K1 on human lung cancers and has shown that B3 reacts with many nonsmall cell carcinomas of the lung, whereas, K1 does not. K1 reacts with mesotheliomas, whereas, B3 does not. Therefore, these two antibodies can be used in the differential diagnosis of lung tumors.